1. Field of the Invention
The present invention relates to a cathode for an electron tube and a preparing method therefor, and more particularly, to a cathode for an electron tube having enhanced life span and electron emission characteristics and a preparing method therefor.
2. Description of the Related Art
In general, an oxide cathode is widely used as a cathode for an electron tube, the oxide cathode having an emitter made of an oxide converted from an alkaline earth metal carbonate salt having barium (Ba) as a main component, on a base metal containing nickel (Ni) as a main component and a small amount of silicon (Si) or magnesium (Mg) as a reducing agent.
Thus, the life span characteristic of an oxide cathode is largely affected by a base metal and an oxide. In particular, an intermediate product generated during the operation of an oxide cathode prevents inhibits diffusion of a reducing agent and thus greatly limits the life span of the cathode, which will now be described in detail.
FIG. 1 is an optical microphotograph of the surface structure of a conventional base metal, that is, nickel having 0.05 wt % of Si and 0.05 wt % of Mg, in which the particle size in the surface structure is approximately 100 xcexcm. The oxide cathode is fabricated as follows. First, carbonate salt powder having barium carbonate as a main component is mixed with an organic solvent prepared by dissolving nitrocellulose and the mixture is deposited on a base metal by a spraying or electrolytically depositing method to then be mounted on an electron gun for being assembled in an electron tube. The carbonate salt is heated to approximately 1000xc2x0 C. by a heater during an exhaustion step for making the inside of the electron tube into a vacuum state, during which barium carbonate is converted into barium oxide.
BaCO3xe2x86x92BaO+CO2xe2x80x83xe2x80x83(1) 
During cathode operation, the thus generated barium oxide reacts with a reducing agent, Si or Mg, contained in the base metal at the interface where the base metal contacts an electron-emitting material layer.
BaO+Mgxe2x86x92MgO+Baxe2x80x83xe2x80x83(2) 
xe2x80x834BaO+Sixe2x86x92Ba2SiO4+2Baxe2x80x83xe2x80x83(3)
The formed free Ba contributes to electron emission. As expressed in the formulas (2) and (3), MgO, Ba2SiO4 or the like is also formed in the interface between the electron-emitting material layer and the base metal or in the particle boundary of the base metal. The reaction product serves as a barrier called an intermediate product to thus prevent diffusion of Mg or Si, thereby making it difficult to generate free Ba which contributes to electron emission. Also, the intermediate product undesirably results in shortening of the life span of the oxide cathode. Further, the intermediate product has high resistance and prevents the flow of current for emitting electrons and thus limits current density.
To overcome these problems, a cathode prepared by depositing a metal layer made of tungsten (W), molybdenum (Mo) and the like on a base metal has been proposed in Japanese Laid-open Patent Publication No. Hei 3-257735 by Matsushita Electric Industrial Co., Ltd. However, the proposed cathode generates additional intermediate product Ba3WO3 as well as a reducing agent. Thus, initial electron emission is excessive and the electron emission and life span characteristics of the cathode decreases over time.
To solve the above problems, it is an object of the present invention to provide a cathode for an electron tube, which can solve problems of shortened life span, increased cut off drift characteristics and so on, caused by intermediate products generated in the interface between a base metal and an electron-emitting material layer during the operation of the cathode and in the particle boundary of the base metal, and a preparing method therefor.
Accordingly, to achieve the above object, there is provided a cathode for an electron tube having a base metal and an electron-emitting material layer, wherein the particle size of the micro structure of the surface of the base metal is controlled to be in the range of 3 to 50 xcexcm.
Preferably, the particle size of the micro structure of the surface of the base metal is controlled by thermal treatment.
The thermal treatment is performed by the steps of:
(a) an oxidative thermal treatment step of heating a base metal at a temperature of 300 to 1100xc2x0 C. under the atmosphere to form a metal oxide layer;
(b) a dry reducing thermal treatment step of heating the base metal having the metal oxide layer at a temperature of 500 to 1200xc2x0 C. under a hydrogen atmosphere in which a dew point is kept at xe2x88x9250 to xe2x88x9290xc2x0 C., to remove the metal oxide layer; and
(c) a wet reducing thermal treatment step of heating the base metal treated with the step (b) at a temperature of 500 to 1200xc2x0 C. under a hydrogen atmosphere in which a dew point is kept at xe2x88x9210 to xe2x88x9240xc2x0 C.
The oxidative thermal treatment step is maintained at the uppermost temperature for 3 to 60 minutes.
Also, the dry reducing thermal treatment step is maintained at the uppermost temperature for 3 to 60 minutes.
Further, the wet reducing thermal treatment step is maintained at the uppermost temperature for 3 to 60 minutes.